Until now, a propulsion engine which uses oil as fuel has been generally used in a merchant ship such as a container carrier or a passenger ship. Due to the recent oil price increase, however, more and more ships are employing a propulsion engine which uses liquefied fuel gas, for example, liquefied natural gas (LNG), which is much cheaper than oil. Furthermore, since the price of LNG in the summer season is 50% lower than in the winter season, LNG may be purchased and stored in the summer season. Therefore, LNG has an advantage in terms of price.
A ship engine capable of obtaining a propulsion or generation power by using LNG as fuel may include a ME-GI engine or a dual fuel(DF) engine. The ME-GI engine is referred to as a gas injection engine which compresses LNG and then injects and burns the compressed LNG. In particular, the ME-GI engine is referred to a high-pressure gas injection engine which compresses LNG, or LPG at a high pressure of 150-600 bar and then injects and burns the compressed LNG, or liquid LPG.
Such a gas injection engine has an operation structure which uses both oil (heavy fuel oil (HFO) or marine diesel oil (MDO)) and fuel gas (LNG, or LPG), and obtains a power by supplying oil to the engine at a low power (for example, less than 30% of the maximum power) and obtains a power by using both oil and fuel gas when the power level is equal to or more than 30% of the maximum power. The operation method of the gas injection engine has been disclosed in Korean Patent No. 0396471.
Such a gas injection engine has a problem in that fuel gas which is a cheap and clean energy source cannot be used at a low power. In general, a ship sails at a low power around a harbor. When the ship uses oil as an energy source at a low power, large amounts of environmental pollutant materials such as SOx and NOx are discharged. Recently, more and more countries tend to set up storing regulations against the discharge of environmental pollutant materials around a harbor.
Therefore, there is a demand for a new propulsion system which is capable of using fuel gas, which is cheap and produces small amounts of environmental pollutant materials, as fuel even at a low power.